There are many inorganic and organic water retentive substances, i.e, substances having high water retention capacity. Okara is one of the representative organic water retentive substances. Okara is the insoluble residue from "tofu", soymilk, soybean protein production and is mainly composed of polysaccharides (e.g., cellulose, hemicellulose, etc.) with a small amount of oligosaccharides and/or fats and oils. Recently, in view of its good water retention characteristics, dried okara has been used as a food additive.
On the other hand, although various drying methods and dryers have been known, they are still insufficient for drying a water retentive substance such as okara without deterioration of its properties and quality.
For example, although it has been known that a substance having a high water content can be dried by using a drum dryer, it is difficult to dry a water retentive substance without deterioration of its properties and quality, especially, without deterioration of its water retention capacity and whiteness by using a drum dryer. Further, there is such a mechanical problem as wear of a blade for scraping a dried product adhered to a drum (see, e.g., Japanese Laid Open Publication No. 148562/1978). It has been also known that a water retentive substance having a high water content can be dried without deterioration of its properties and quality by using a spray dryer. However, in case of a water retentive substance having a low water content, a high viscosity and/or such a large particle size that a nozzle is clogged, it is difficult to dry the substance by using a spray dryer. Further, it is also difficult to dry a substance having a high water content (about 40% or more) by using a known pneumatic conveying dryer, and properties and quality of a water retentive substance are liable to be deteriorated by using such a dryer.
Under these circumstances, the present inventors have intensively studies to find out an efficient drying method of a water retentive substance, especially, okara without deterioration of its properties and quality such as water retention capacity, color tone, etc. As the result, it has been found that a certain kind of a pneumatic conveying drying apparatus is especially suitable for drying a water retentive substance such as okara without deterioration of its properties and quality.
Recently, an improved pneumatic conveying dryer has been developed. This dryer comprises a pneumatic conveying dryer section, a pulverizer section provided at the lower part of the dryer section, a hot gas inlet provided below the pulverizer section, a classifier section provided at the upper part of the dryer section and an exhaust vent connected to the upper end of the classifier. In this dryer, a hot gas flows from the inlet upwardly through the pulverizer and the dryer sections and a substance to be dried is fed to the dryer section through a feeder opening. Firstly, the substance fed falls under the influences of gravity to the pulverizer section. Then, the substance is pulverized and then whirled up toward the classifier section through the dryer section by means of the hot gas flow. The substance is dried during this process. Principally, this dryer is used for drying a hard substance or a substance which requires pulverization (generally, a substance having a low water content). Further, it is very difficult to use this dryer for drying a water retentive substance having a high water content. In fact, when using this dryer for drying okara which is a plastic substance having a high water content, okara adheres to the pulverizer section and the inner surface of the dryer section, which makes the drying difficult. In addition, even if okara is forced to be dried by using this dryer, the properties and quality of the resultant dried okara are deteriorated, for example, lowering of the water retention capacity and browning result.
However, it has been surprisingly found that such a dryer can be efficiently used for drying a water retentive substance having a high water content such as okara by making some improvements based on the following present inventors' finding:
(1) In this dryer, because a substance to be dried is fed to the dryer section at the neighborhood of the pulverizer section, adhesion of the substance such as okara to the inner surface of the dryer section is unavoidable and the properties and quality of the substance (water retention capacity, color tone, etc.) are deteriorated;
(2) When the substance to be dried is fed to the dryer section at a site at least 500 mm above the pulverizer section, adhesion can be prevented;
(3) When the shape of the substance to be dried is adjusted to particles having particle size of 1.5 to 13 mm which is finer than that conventionally employed, a dried product having excellent properties and quality can be efficiently obtained;
(4) When the water content of a substance to be dried is adjusted to 55 to 95% by weight, in case of okara, preferably, 55 to 85% by weight, more preferably, 65 to 75% by weight, adhesion of the substance to the inner surface of the dryer section is prevented and a dried product having excellent properties and quality can be efficiently obtained; and
(5) Adhesion of the substance can be also prevented by covering the inner surface of the apparatus with an anti-adhesion region such as a fluorocarbon resin.